1. Field of the Invention
The present invention relates in general to a hydraulic booster used for a hydraulically operated braking system for a motor vehicle or the like, and more particularly to such a hydraulic booster which is normally operated by an external hydraulic power source and which is equipped with an emergency accumuator whose pressure is applied to the booster upon failure or trouble of the external hydraulic power source.
2. Discussion of the Prior Art
The hydraulic booster generally includes a housing, a reaction piston, a power piston and a control valve. The reaction and power pistons are slidably fitted or received in the housing, such that a power chamber is defined between the reaction and power pistons. A power pressure in the power chamber is regulated by the control valve which is operated in response to an operation or movement of the reaction piston. The power piston is activated by the thus regulated power pressure. The pressure receiving area of the power piston is made smaller than that of the reaction piston, so that an output force of the power piston is greater than an operating force which acts on the reaction piston.
One known type of hydraulic booster is actuated by a working fluid which is normally recirculated by a pump through the system, under a relatively low pressure. While the reaction piston is at rest, the fluid delivered from the pump is merely passed through a normally open passage in the control valve, and returned to a reservoir of the system. As the reaction piston is operated, the control valve is operated so as to restrict a flow of the fluid through the control valve, whereby the fluid pressure produced by the pump is raised, and a power chamber passage formed through the control valve is opened so that the raised fluid pressure is applied to the power chamber through the opened chamber passage, to activate the power piston.
Where the hydraulic booster of the type described above is applied to a hydraulic braking system for an automotive vehicle, a hydraulic power source for a power steering device of the vehicle may be utilized as a power source for the hydraulic booster. This utilization is possible, since a common power steering device is adapted such that the fluid pumped up from the reservior by the pump is returned to the reservoir through a power steering valve, under a low pressure, while the power steering device is in a non-operated position. However, in the event of a pressure failure of the hydraulic power source of the power steering device, the hydraulic booster cannot provide a normal boosting function, necessarily reducing the braking capacity of the braking system.
To avoid such a situation, it is proposed to use an emergency accumulator as disclosed in S. A. E. Report 730536 (published in 1973, from Society of Automotive Engineers, Incorporated). The proposed emergency accumulator is connected to the control valve through an accumulator passage provided with a check valve. The emergency accumulator is adapted to store a portion of the fluid whose pressure is elevated by the control valve. The stored pressurized fluid is supplied to the power chamber of the hydraulic booster to activate the power piston, in the event of a failure or trouble of a pump or other elements of the hydraulic power source of the hydraulic booster. Thus, the emergency accumulator enables the hydraulic booster to function to effect a normal boosting operation, until the fluid pressure in the accumulator is sufficiently high. The use of the emergency accumulator improves the reliability of the hydraulic braking system of the vehicle.
However, the known hydraulic booster equipped with the emergency accumulator indicated above suffers from a problem that the pressure of the fluid stored in the accumulator is equal to or lower than the maximum pressure level of the fluid which is supplied to the power chamber of the hydraulic booster during an operation of the booster. Although it is desirable that the fluid pressure in the accumulator be higher than the pressure of the fluid to be supplied to the power chamber, the upper limit of the pressure range of the fluid stored in the accumulator cannot exceed the fluid pressure applied to the power chamber. It is also noted that the fluid pressure in the accumulator is lowered as the stored fluid is fed to the hydraulic booster. For the above reasons, the hydraulic booster cannot operate to perform a required boosting function, in the event of a failure of its hydraulic power source, even where the emergency accumulator is provided.